1. Field of the Invention
This invention relates to a method and a circuit for balancing energy levels among batteries in a battery pack.
2. Disclosure of Related Art
As shown in U.S. Pat. No. 5,631,534, a conventional system for powering an electric or hybrid vehicle may include a battery pack having a plurality of batteries connected in series across a first current bus. The first current bus may be used to provide power to an electric motor in the vehicle. The power provided across the first current bus is limited by the energy level of the weakest battery in the group of series connected batteries. Accordingly, also as shown in U.S. Pat. No. 5,631,534, it is conventional to provide a circuit for balancing the energy levels among the series connected batteries. It is also conventional for the battery pack to include an additional battery coupled across a second, accessory current bus that provides power for lighting, starting, and other electrical systems in the vehicle. The use of an additional battery to power vehicle electrical systems increases the cost and size of the battery pack and vehicle, however.
There is thus a need for a method and circuit for balancing energy levels in a battery pack that will minimize or eliminate one or more of the above-mentioned deficiencies.